1. Field of the Invention
The present invention relates to an electronic timer circuit and in particular to a new and useful programmable electronic timer circuit. The timer circuit is designed to provide stable, accurate and repeatable time delays between an input signal it receives and an output signal it produces over a wide range of operating voltages and temperatures. The invention also relates to an electronic detonator circuit that includes such a timer circuit to provide an output signal to initiate an explosive charge after a predetermined time interval from receipt of an initiation signal.
2. Related Art
In a detonator for initiating an explosive charge it is often important to precisely control the timing with which the explosive charge is initiated after receipt of an initiation signal. It is known to provide a detonator with a pyrotechnic or an electronic timer for this purpose. For example, in controlling the timing of a sequence of explosions in blasting operations such as mining, quarrying, construction or demolishing a structure, such as a building, a series of explosive charges must be set off in a precisely timed sequence, in order to obtain the desired blasting effect, minimize shock forces acting on the surrounding area and properly demolish the structure. This requires a series of detonators, each of which can initiate an explosive charge at a predetermined precise time interval, usually measured in milliseconds, from receipt of an ignition signal.
Conventional pyrotechnic delay elements incorporated into detonators used to initiate explosive charges are subject to inherent manufacturing variations with respect to density and type of chemical delay composition contained therein, and so cannot be relied upon to provide highly accurate delay intervals.
It is known in the art that electronic time delay circuits can be used in place of pyrotechnic delay elements. For example, the inclusion within a conventional-sized detonator cap of electronic timing circuitry in lieu of the usual pyrotechnic delay train to provide a delay between receipt by the cap of an initiation signal and detonation of the cap, is illustrated in U.S. Pat. No. 5,173,569 of Robert G. Pallanck et al, issued Dec. 22, 1992, for "Digital Delay Detonator". This patent shows a detonator cap that incorporates electronic circuitry which is responsive to an input signal to the cap to establish a delay between receipt of the input signal and detonation of a small explosive charge within the cap. The cap is mounted on the end of a length of shock tube which carries an impulse type initiation signal to the cap. The impulse signal acts on a piezoelectric generator forming part of the circuitry, and the piezoelectric generator generates an electric input signal to the electronic timer circuit. After a predetermined delay the timer circuit emits an output signal that is used to fire the cap.
Conventional electronically-timed detonators suffer from limitations inherent in conventional electronic timers with respect to the flexibility and reliability with which they may be programmed (to provide a desired delay interval) and tested. For example, conventional multi-stage digital timers may consist of a number of toggle-counter stages, each with a separate line that is brought out of the circuit for programming purposes. Each of those lines has to be mechanically connected to either the supply voltage or ground signal and another program line is required to load these program signals into the counter stages. The counter stages are pre-set to the voltage levels that their individual program lines are connected to when the program line is activated. Such timers do not contain built-in voltage regulators and do not contain built-in oscillator circuits. A conventional fourteen-stage programmable counter would require two power supply lines, fourteen programming lines, one program load line, one oscillator input line and at least one output line. Such a circuit would require at least nineteen separate lines for proper operation.